(i) Field of the Invention
The present invention relates to a porous film and a preparation process thereof. More specifically, the present invention relates to a porous film in which moisture vapor transmission, feeling, uniformity of film thickness and the like are maintained equally to their conventional levels and adhesive properties are improved, and a preparation process thereof.
(ii) Description of the Prior Art
Heretofore, there have been proposed many methods in which each film comprising a polyolefin resin and an inorganic filler is stretched in uniaxial or biaxial directions to generate continuous voids in the film itself, thereby manufacturing porous films. The porous films have been used for many uses such as sanitary materials, medical materials, clothing materials, building materials and separators for cells and the like. However, any porous films having flexible feeling cannot be obtained by the blend system comprising the polyolefin resin and the inorganic filler alone, and hence the conventional porous films cannot be used in fields in which soft feeling like cloth, the flexible feeling and the like of films, are required. As a method which can solve such a problem, there has been proposed a method which comprises adding an additive such as a fatty acid ester as a third component to the blend system comprising the polyolefin resin and the inorganic filler.
For example, Japanese Patent Application Laid-open No. 18435/1987 has disclosed a method for preparing a breathable film which comprises stretching a film formed from a composition of 42 to 87% by volume of a polyolefin resin and 58 to 13% by volume of an inorganic filler in at least uniaxial direction, the composition being blended with an aliphatic alcohol-based fatty acid ester obtained from a fatty acid having 10 to 22 carbon atoms and an aliphatic alcohol having 1 to 12 carbon atoms in an amount of 3 to 25 parts by weight based on 100 parts by weight of the composition.
In this publication, it is described that examples of the fatty acid having 10 to 22 carbon atoms include lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, linolic acid, linolenic acid and ricinolic acid, and above all, ricinolic acid is most preferable. Furthermore, it is also described that examples of the aliphatic alcohol having 1 to 12 carbon atoms include methyl alcohol, ethyl alcohol, butyl alcohol, ethylene glycol, propylene glycol, glycerin, pentaerythritol and sorbitol, and above all, glycerin is most preferable. In an example of the publication, it is described that a purified castor oil containing 85% or more of glycerin triricinolate was used (hereinafter referred to simply as a purified castor oil).
Moreover, Japanese Patent Application Laid-open No. 227738/1992 has disclosed a stretched film containing a filler which comprises a resin composition consisting of (a) a linear low-density polyethylene copolymer having a density of 0.910 to 0.945 g/cm.sup.3 and a melt flow rate of 0.01 to 20 g/10 min, and containing 1 to 20% by weight of a comonomer of an .alpha.-olefin or a diolefin having 6 or more carbon atoms, and the copolymer having an extraction quantity (by boiling normal hexane) of 20% by weight or less, (b) a particulate inorganic filler having an average particle diameter of 10 .mu.m or less and a bulk density of 0.1 to 0.7 g/cm.sup.3, (c) a saturated or an unsaturated fatty acid ester having 9 to 40 carbon atoms, the blend ratio of the component (c) being in the range of 0.1 to 15 parts by weight based on 100 parts by weight of 20 to 80% by weight of the component (a) and 80 to 20% by weight of the component (b), and a MD tensile strength of the film being 10 g or more.
It is also described in the publication that, as the component (c), the saturated fatty acid ester is preferably glyceryl hydroxystearate, more preferably glyceryl-12-hydroxystearate, and the unsaturated fatty acid ester is preferably glyceryl ricinolate. In examples of the publication, as the former ester, 2 to 3% by weight of a castor wax was used, and as the latter ester, 1 to 3% by weight of a purified castor oil was used.
According to investigation by the present inventors, with regard to a porous film obtained by using, as a third component, the purified castor oil described in Japanese Patent Application Laid-open No. 227738/1992, some facts have been elucidated. For example, when an adhesive tape having an adhesive layer such as a securing tape is stuck on the surface of this porous film via the adhesive layer, the purified castor oil migrates to the adhesive layer through the surface of the porous film with time, and as a result, the adhesive strength of the adhesive layer deteriorates, so that the securing tape is finally separated from the porous film. In addition, it has also been grasped that the above-mentioned phenomenon cannot be suppressed even in the case that the blend amount of the purified castor oil is 1 part by weight with respect to 100 parts by weight of the resin composition which is the lower limit of the use range of the castor oil described in examples of this publication.
Therefore, when the porous film prepared by the above-mentioned method is used as, for example, the back sheet of a disposable diaper to attach this disposable diaper to a human body, it is difficult to fix the diaper to the body by the securing tape or the like, which causes a problem that human urine leaks out.
Furthermore, with regard to the porous film obtained by using, as the third component, an ester such as the castor wax which is solid at room temperature, its adhesive strength scarcely deteriorates with time, but the obtained porous film is poor in softness.
Japanese Patent Application Laid-open No. 32136/1987 has disclosed a high filling composition comprising (a) 5 to 50% by weight of an ethylene-.alpha.-olefin copolymer having a melt index (MI) of 0.1 to 100 g/10 min and a density of 0.86 to 0.94 g/cm.sup.3, (b) 0.1 to 15% by weight of ester acids consisting of a polyhydric alcohol and a saturated and/or an unsaturated aliphatic monobasic acid having 2 to 24 carbon atoms and a hydroxy acid, and (c) 30 to 90% by weight of an inorganic filler. In the disclosed invention, however, the composition is used as a lining material of a carpet for a car or a tile carpet, and for the purpose of imparting a sound insulation effect to the carpet, the inorganic filler is added at a high concentration to increase the weight of the inorganic filler per unit area of the carpet. Therefore, the disclosed invention is quite different in a technical field from the porous film of the present invention which is useful for a use such as the disposable diaper having the problem of the adhesive strength deterioration of the securing tape or the like. In addition, there has not been described a mixing ratio of the plural ester acids comprising the polyhydric alcohol and the saturated and/or the unsaturated aliphatic monobasic acid having 2 to 24 carbon atoms and the hydroxy acid. Moreover, it has not been disclosed anywhere in the publication that the porous film having a suitable softness and excellent adhesive properties can be obtained by mixing these esters with a polyolefin resin in a specific ratio as in the present invention.